The invention relates generally to a radio occultation instrumentation system and, more particularly, to open loop tracking of low altitude signals.
Radio Occultation instruments using Global Navigation Satellite System (GNSS) signals are space borne receivers, which provide information, regarding the Doppler shift of signals transmitted from, for instance, a Global Positioning System (GPS) satellite having a well defined position in a well-defined orbit and a transmitter with a well-defined frequency. The signal from the GPS satellite is received, after having crossed the atmosphere, by a receiver at a Low Earth Orbiting satellite. The Doppler shift of the received signal is measured and from this measurement, vertical profiles of the temperature, pressure and density in the atmosphere can be derived. Thus, the basic function of a Radio Occultation instrument is to receive and acquire signals that have crossed the atmosphere at varying altitudes, even if such signals have crossed dense tropospheric layers, which often have large refractivity causing dynamics in amplitude and phase of the signal.
GNSS radio Occultation as an atmospheric sensing tool has advantages regarding all weather capability because clouds do not block the signals. The method provides for high vertical resolution from 1.5 km in the stratosphere to 0.2 km in the troposphere. The accuracy of the retrieved temperature is in the range of 1K. Furthermore the method provides for long-term consistency, which is essential for climate change monitoring.
An illustration of the use of Radio Occultation with respect to a setting satellite is described below with reference to FIG. 1.
An instrument, e.g., in satellite 3 (the setting satellite), acquires and tracks a transmitted GPS signal from satellite 2 at two frequencies L1 and L2. The transmitted signal is represented by a ray path 1 from the satellite 2 to the low earth orbiting satellite 3 and, initially, does not pass the upper parts of the atmosphere. This position is indicated with P1 in FIG. 1. When the ray path 1 descends into the atmosphere, the instrument measures the carrier phase and the Doppler shift is calculated from the measured carrier. The phase measurements are transmitted from satellite 3 to a receiving station 4 positioned on the earth. The Doppler shift recorded together with information regarding the position and the velocity of the satellites is used for determining the directions of the reception and transmission. The position of the satellite 2 carrying the transmitter is denoted by xG and the position of the low earth orbiting satellite 3 is denoted by xL, where xG and xL are vectors. A further vector x is defined as the positions of both satellites, that is x=(xG, xL). Turning briefly to FIG. 2, illustrations of ray geometry are shown. In the case of a spherically symmetric atmosphere the central part of the ray path, that is a portion 5 of the ray path around the point where the distance to the earth is at a minimum, is symmetric. Therefore the distances, known as impact parameters a, from the earth centre to ray asymptotes 6,7 at the respective impact point 8,9 are equal. The impact points 8,9 are situated on the ray path 1 where a plane perpendicularly arranged to a ray asymptote 6,7 passes through the centre of the earth. Furthermore the ray asymptotes cross each other at an angle xcex1, which is defined as the refraction angle. The impact parameters a, and the refraction angle xcex1, can be calculated from the Doppler shift using only measurement geometry. Thus, the refraction angle xcex1 can be derived as a function of the impact parameter. When performing measurements, the dispersive behaviour of the ionosphere and the non-dispersive behaviour of the neutral atmosphere is used to enable determination of the refraction angle contribution from each of these layers. Further, use of the symmetric atmosphere condition allows for determination of the refraction index profile xcex1(r) as a function of the earth radius from the refraction angle xcex1 by using the Abel transform. The refraction index profile xcex1(r) depends on the air pressure, temperature and water vapour content, which parameters are retrieved using the gas equation and the hydrostatic equilibrium equation.
A more detailed presentation of how radio occultation is performed is provided in P. Hoeg et al., xe2x80x9cThe Derivation of Atmospheric Properties by Radio Occultationxe2x80x9d, Danish Meteorological Report 95-4, 1994, E. R. Kursinsky et al., xe2x80x9cObserving Earth""s Atmosphere with radio Occultation Measurements using the GPSxe2x80x9d, Journal of Geophysical research, 102, no. D19 Oct. 1997, W. G. Melbourne et al., xe2x80x9cThe Application of Spaceborne GPS to Atmospheric Limb Sounding and Global Change Monitoringxe2x80x9d, JPL Publication 94-18, 1994 and C. Rocken et al., xe2x80x9cAnalysis and validation of GPS/MET Data in the Neutral Atmospherexe2x80x9d, Journal of Geophysical Research, 1998, which all are incorporated by reference.
At high altitudes, where the signal is strong, the receiver locks on to the signal carrier. However, as the ray path transverses the atmosphere at lower altitudes, the ray is gradually more bent, attenuated and spread, due to the increase in refractivity. Consequently, tracking can sometimes not be performed at the lower altitudes, where the signal is weak and has large dynamic properties. Therefore, there is a need for an aided tracking that is model based. While, attempts have been made to perform model based tracking, the suggested methods have difficulties with the atmospheric Doppler shift at the lowest altitudes. This has resulted in loss of signal or inaccuracies in the measurement results for low altitudes (altitudes less than 5 km above the surface of the earth).
An object of the invention is to provide apparatus for performing open loop tracking of a signal in a radio occultation instrumentation system, where the effects of the large variation of the atmospheric Doppler shift at low altitudes are mitigated. Therefore, and in accordance with the invention, a satellite receiver tracks a received signal by predicting a Doppler shift displacement of the received signal as a function of the distance of an impact point a relative to the earth, the positions of a first and a second satellite and the velocity of the first and the second satellite.
In an illustrative embodiment of the invention, a satellite receiver comprises a frequency control unit and a means providing parametric value Q(x,v,xcex1(x)), which corresponds to Doppler shift displacement. The parametric value Q(x,v,xcex1(x)) asymptotically converges towards a function F(x,v) dependent on the satellite positions and velocities and independent of atmosphere conditions when a ray path leaves the atmosphere, and asymptotically towards a fixed value when the ray path approaches the surface of the earth, wherein the parametric value is arranged for providing signal acquisition by the frequency control unit.
In another embodiment of the invention, the above-described Doppler shift displacement is used in a method for determining atmospheric conditions.
In particularly preferred embodiments a straight-line tangential altitude (SLTA) is used as a basis for the calculation of the parametric value. This is particularly advantageous since the SLTA continues to decrease to large negative numbers when the impact altitude asymptotically converges to a constant value at low impact altitudes, and thereby the effects of a large variation in atmospheric Doppler can be compensated.